With the increasing popularity of computers, and with the advent of mass networks such as the Internet, electronic distribution of content, such as documents, images, and sound, has become much more common. However, electronic content distribution has raised concerns for the creators of such content. For example, content creators wish to ensure that copyright and other authorship and ownership and usage information is attached to their electronic content, such that subsequent attempts to determine the author, owner and user of the content are successful. Furthermore, the creators wish to determine that their content has not been altered since first being distributed electronically.
Watermarking is a technology designed to address the concern of digital content owners and providers who wish to distribute their products and material on the Internet or by other means that their content, which includes text images, and video, may be copied, altered and distributed without permission. Digital watermarking inserts information in digital content that can be recovered even if the watermarked content is altered with significant degradation or alteration.
Watermarking allows questions of ownership and use of a given piece of content—which may be widely distributed by virtue of the Internet, for example—to be resolved, by attempting to decode an embedded secret from the content. That is, by watermarking content data, the data owner can determine whether a suspect piece of content is his or hers by determining whether the watermark is present in the suspect data. Watermarking is a technique used to label digital content by hiding copyright or other information into the underlying data. Unlike encryption used to restrict access to data, watermarking can be employed to provide solid evidence of authorship and usage. Like data hiding generally, the watermark remains with the media through typical content-preserving manipulations, e.g. cropping, compression, and so forth. However, unlike data hiding generally, with watermarking an unauthorized user cannot access the embedded information (e.g. the watermark). In addition, the power of an effective watermarking system lies in its degree of robustness. Robustness ensures that the embedded watermark can not be removed or tampered without destroying or at least degrading the quality of the information.
Generally the information embedded in the watermark is a pattern whose location is known by the watermark detector. Robust watermarks can be detected following operations that do not change the position of the watermark such as the addition of noise and blurring. However, detectors that must know the position of the watermark cannot detect it following operations that change its position. Common operations that change the position of a watermark are shifts and scaling. Images can be scaled and shifted following printing and scanning and videos can be scaled and shifted following analog to digital conversion.
A template is a secondary watermark that can be used to synchronize the detector with the primary watermark. Templates like other watermarks can be embedded in the spatial domain or the frequency domain. A template that is embedded in the frequency domain using the Fourier transform increases the magnitude of Fourier coefficients as disclosed by Shelby Pereira, Joseph J. K. O' Ruanaidh and Thierry Pun, “Secure robust digital watermarking using the lapped orthogonal transform, Society of Photo-optical Instrumentation Engineers, Vol. 3657, January 1999, pp. 21–30, which is incorporated herein. Templates produced by this method tend to become visible for images and video the size used by MPEG1 or smaller. Visibility is a problem for a template because a visible template is more easily detected, and then thwarted. Templates implemented in the frequency domain are difficult to the control artifacts, and are easy to distort through common image processing operations such as blurring and low-bit rate compression.
For the reasons stated above, and for other reasons stated below which will become apparent to those skilled in the art upon reading and understanding the present specification, there is a need in the art for an invisible watermark that in the spatial domain that can decoded even after scaling and shifting of the image.